Viral Infections Activate Types I and III Interferon Genes through a Common Mechanism*

Viral infections trigger innate immune responses, including the production of type I interferons (IFN-α and -β) and other proinflammatory cytokines. Novel antiviral cytokines IFN-λ1, IFN-λ2, and IFN-λ3 are classified as type III IFNs and have evolved independently of type I IFNs. Type III IFN genes are regulated at the level of transcription and induced by viral infection. Although the regulatory mechanism of type I IFNs is well elucidated, the expression mechanism of IFN-λs is not well understood. Here, we analyzed the mechanism by which IFN-λ gene expression is induced by viral infections. Loss- and gain-of-function experiments revealed the involvement of RIG-I (retinoic acid-inducible gene I), IPS-1, TBK1, and interferon regulatory factor-3, key regulators of the virus-induced activation of type I IFN genes. Consistent with this, a search for the cis-regulatory element of the human ifnλ1 revealed a cluster of interferon regulatory factor-binding sites and a NF-κB-binding site. Functional analysis demonstrated that all of these sites are essential for gene activation by the virus. These results strongly suggest that types I and III IFN genes are regulated by a common mechanism.